A telescopic shaft that is composed of spline shafts that are capable of transmitting rotation torque and are capable of relative sliding movement in the axial direction is assembled in a steering apparatus as an intermediate shaft, steering shaft or the like. More specifically, for an intermediate shaft, when connecting the universal joint thereof to a pinion shaft that engages with a rack shaft of a steering gear, it is necessary to fit the intermediate shaft itself with the pinion shaft after contraction the intermediate shaft, and furthermore, it is necessary to absorb the relative displacement between the intermediate shaft and the vehicle frame, the intermediate shaft is provided with a telescopic function. Moreover, a steering shaft is provided with a telescopic function for transmitting steering force from a steering wheel to the wheels and allowing the position of the steering wheel in the axial direction to be adjusted to match the physique and driving posture of the driver.
FIG. 15 illustrates an example of a conventional steering apparatus having this kind of telescopic shaft. This steering apparatus comprises: a steering shaft 12 onto which a steering wheel 11 can be mounted on the side toward the rear of the vehicle; a steering column 13 that with the steering shaft 12 inserted through it, supports the steering shaft 12 so as to be able to rotate; an assist apparatus (auxiliary steering unit) 20 for applying auxiliary torque to the steering shaft 12, and on the side toward the front of the vehicle of the steering shaft, an intermediate shaft and a steering gear 30 that are connected by way of a rack and pinion mechanism (not illustrated in the figure).
The steering shaft 12 is constructed by combining an outer shaft 12A and an inner shaft 12B such that rotation torque can be freely transmitted, and so that relative displacement in the axial direction is possible. In order for this, a plurality of male splines is formed around the outer circumference of the inner shaft 12B on the side toward the rear of the vehicle. A plurality of female splines is formed around the inner circumference of the outer shaft 12A on the side toward the front of the vehicle having the same phase positions as the male splines. By fitting the female splines of the outer shaft 12A with the male splines of the inner shaft 12B with a fit having a specified clearance gap, the outer shaft 12A and the inner shat 12B are fitted together such that rotation torque can be freely transmitted, and relative displacement in the axial direction is possible. With this construction, the outer shaft 12A and the inner shaft 12B are capable of relative sliding movement during a collision, such that the entire length of the steering shaft 12 is able to contract.
Moreover, a cylindrical steering column through which the steering shaft 12 is inserted is constructed by combining an outer column 13A and an inner column 13B so as to be able to expand and contract, and comprises collapsible construction wherein when an impact is applied in the axial direction during a collision, the entire length contracts while absorbing energy due to the impact.
The end section on the side toward the front of the vehicle of the inner column 13B is press fitted and fastened to the end section on the side toward the rear of the vehicle of a gear housing 21. Moreover, the end section on the side toward the front of the vehicle of the inner shaft 12B is inserted through the inside of the gear housing 21 and is connected to the end section on the side toward the rear of the vehicle of an input shaft (not illustrated in the figure) of the assist apparatus 20.
The middle section of the steering column 13 is supported by part of the vehicle 18 such as the bottom surface of the dashboard by a support bracket 14. A locking section (not illustrated in the figure) is provided between the support bracket 14 and the vehicle 18 so that when an impact is applied in a direction toward the front side of the vehicle, the support bracket 14 is able to separate from the locking section, and move toward the front side of the vehicle.
The top end section of the gear housing 21 is also supported by part of the vehicle 18. This conventional construction comprises a tilt mechanism and a telescopic mechanism, that make it possible to freely adjust the forward-backward position with respect to the vehicle (telescopic position) and the height position (tilt position) of the steering wheel 11. The construction of the telescopic mechanism and the tilt mechanism is well known, so a detailed explanation will be omitted here.
An output shaft 23 that protrudes from the end surface on the side toward the front of the vehicle of the gear housing 21 is connected to the rear end section of the male shaft 16A of the intermediate shaft 16 by way of a universal joint 15. Furthermore, the pinion shaft 31 of the steering gear 30 is connected to the front end section of the female shaft 16B of the intermediate shaft 30 by way of a different universal joint 17.
The male shaft 16A is connected to the female shaft 16B such that relative sliding movement in the axial direction is possible, and so that rotation torque can be transmitted. A pinion that is formed on the front end section of the pinion shaft 31 engages with a rack of a rack shaft (not illustrated in the figure), and rotation of the steering wheel 11 causes tie rods 32 to move by way of the steering gear 30, and steers the wheels (not illustrated in the figure).
The case 261 of an electric motor 26 is fastened to the gear housing 21 of the assist apparatus 20. The direction and size of the torque that is applied to the steering shaft 12 from the steering wheel 11 is detected by a torque sensor, and the electric motor 26 is driven according to this detected signal, making it possible to apply an auxiliary torque having a specified direction and specified size to the output shaft 23 by way of a reduction gear (not illustrated in the figure).
In order to sufficiently take full advantage of the expanding and contracting function in the axial direction of the telescopic shaft such as the intermediate shaft 8 and steering shaft 12, there is a need to reduce the sliding friction that occurs when there is sliding in the axial direction. On the other hand, in this kind of steering apparatus, when the steering wheel 11 is operated as the rigidity and operating stability of the entire vehicle is improved, it becomes easy for the driver to feel backlash movement in the direction of rotation of the telescopic shaft such as the intermediate shaft 16 or the steering shaft 12. Therefore, there is a need for a telescopic shaft that has both reduced backlash in the direction of rotation and the sliding friction, and that has excellent lubrication and durability.
For example, manufacturing of telescopic shaft is being performed wherein a coated section is provided by coating a synthetic resin or the like having small sliding friction on the outer circumference of the tooth surface of the male shaft, and furthermore, after applying a lubricant such as grease, the male shaft is fitted into the female shaft. By providing this kind of coated section, it is possible for the telescopic shaft to slide smoothly due to the self lubrication and flexibility of the resin material. However, in this kind of telescopic shaft, when the allowable range of the sliding resistance is narrow, it may become difficult to keep the sliding resistance within the allowable range due to the processing precision of the male shaft, female shaft and coated section.
A telescopic shaft disclosed in JP 2004-66970 (A) is such that when a female shaft is fitted with a male shaft in which a coated section as been provided, a rotation torque is applied by relatively twisting the male shaft and female shaft such that the inner teeth of the female shaft press against the coated section of the outer teeth of the male shaft, which causes the coated section to be compressed and hardened, and forms pressure dented surface on the coated section. As a result, a sliding clearance gap is maintained at a fixed amount over a long period of time, and by accumulating lubricant in the pressure dented surface, the supply of lubricant is performed well. However, in this telescopic shaft, a specified sliding resistance is obtained near the specified sliding position where rotation torque was applied and the coated section was compressed, however, it is difficult to obtain the specified sliding resistance along the entire length of the distance that the telescopic shaft slides.
As related art, JP 2008-168890 (A) discloses the use of a synthetic resin composition that includes a specified filler as the resin of the coated section, and JP 2011-111091 (A) discloses the use of a synthetic resin composition that has undergone crosslinking treatment as the resin of the coated section. Moreover, JP 2009-168194 (A) discloses a manufacturing method for a telescopic shaft in which the male shaft is caused to slide relatively in the axial direction with respect to the female shaft while heating the coated section to a specified temperature according to the sliding resistance before the intermediate shaft is broken in, in order to keep the sliding resistance with a specified narrow allowable range after the intermediate shaft has been broken in. The contents disclosed in JP 2004-66970 (A), JP 2008-168890 (A), JP 2011-111091 (A) and JP 2009-168194 (A) are incorporated into this specification by reference.